If you're willing to wait, you can often snag excellent components at low prices. 1saleaday.com was selling the 5870 for $130 plus S&H, newegg was selling the 1100T for $150, and a motherboard can be had for around $100. That's three pretty damn good components for $350, and they'll last a good while before needing replacement.
Of course, you may need to wait months for deals like that to come along, so if you want a good rig in the forseeable future, you'll need to take everyone else's advice.